Technical Field
The present disclosure relates to the control of the output power for wireless power transfer systems.
Description of Related Art
Wireless power transfer based on the magnetic resonance and near-field coupling of two loop resonators was reported by Nicola Tesla a century ago. N. Tesla, “Apparatus for transmitting electrical energy,” U.S. Pat. No. 1,119,732, (Dec. 1, 1914). As pioneered by Tesla, wireless power transfer can be radiative or non-radiative depending on the energy transfer mechanisms. Radiative power can be emitted from an antenna and propagates through a medium (such as vacuum or air) over long distance (i.e. many times larger than the dimension of the antenna) in the form of electromagnetic waves. However, due to the omni-directional nature of the radiative power emission, the energy efficiency of power transmission is very low. Non-radiative wireless power transfer relies on the near-field magnetic coupling of conductive loops and can be classified as short-range and mid-range applications. Herein the term mid-range applications refer to the situation where the transmission distance between the power source and the load is larger than the dimension of the coil-resonators.
It should be noted that wireless power transfer has been applied extensively in ac machines, which were also pioneered by Tesla. See, Robert Lomas, “The man who invented the twentieth century—Nikola Tesla—Forgotten Genius of Electricity,” Headline (1999), ISBN 0 7472 6265 9, p. 146. Using a cage induction machine as an example, energy is transferred from the excited stator windings across the air gap to the rotor cage. Energy transfer via coupled windings is the basic principle used in electric machines. Therefore, wireless power systems can be mathematically described by electric circuit theory for magnetically coupled circuits.
Wireless power transfer has been an active research topic for transcutaneous energy systems for medical implants since the 1960's. See, J. C. Schuder, H. E. Stephenson, and J. F. Townsend, “High level electromagnetic energy transfer through a closed chestwall,” IRE Int. Conv. Rec., pt. 9, vol. 9, pp. 119-126, (1961); W. H. Ko, S. P. Liang, and C. D. F. Fung, “Design of rf-powered coils for implant instruments,” Med. Biol. Eng. Comput., vol. 15, pp. 634-640, (1977); E. Hochmair, “System optimization for improved acuracy in transcutaneous signal and power transmission”, IEEE Trans. Biomedical Engineering, vol. BME-31, no. 2, pp. 177-186, (February 1984); B. Choi, J. Nho, H. Cha, T. Ahn and S. Choi, “Design and implementation of low-profile contactless battery charger using planar printed circuit board windings as energy transfer device,” IEEE Trans. Industrial Electronics, vol. 51, no. 1, pp. 140-147, (February 2004); and Y. Jang and M. M. Jovanovic, “A contactless electrical energy transmission system for portable-telephone battery chargers”, IEEE Trans. Industrial Electronics, vol. 50, no. 3, pp. 520-527, (June 2003).
This research has also involved induction heaters since the 1970's. See, W. G. Hurley and J. Kassakian, “Induction heating of circular ferromagnetic plates”, IEEE Trans. Magnetics, vol. 15, no. 4, pp. 1174-1181, (July 1979). For modern short-range applications, the inductive power transfer (IPT) systems have attracted much attention since the 1990's. A. W. Green and J. T. Boys, “10 kHz inductively coupled power transfer-concept and control,” Proc. ICPE-VSD, (1994), pp. 694-699; J. T. Boys, G. A. Covic and A. W. Green, “Stability and control of inductively coupled power transfer systems”, Proc. Electric Power Applications, (2000), vol. 147, no. 1, pp. 37-43; J. T. Boys, A. P. Hu and G. A. Covic, “Critical Q analysis of a current-fed resonant converter for ICPT applications,” Electronics Letters, vol. 36, no. 17, pp. 1440-1442, (2000); G. A. J. Elliott, G. A. Covic, D. Kacprzak and J. T. Boys, “A New Concept: Asymmetrical Pick-Ups for Inductively Coupled Power Transfer Monorail Systems,” IEEE Trans. Magnetics, vol. 42, no. 10, pp. 3389-3391, (2006); and M. L. G. Kissin, J. T. Boys and G. A. Covic, “Interphase Mutual Inductance in Polyphase Inductive Power Transfer Systems,” IEEE Trans. Industrial Electronics, vol. 56, no. 7, pp. 2393-2400, (2009). The wireless charging systems for portable equipment, such as mobile phones have attracted much attention since the 2000's. B. Choi, J. Nho, H. Cha, T. Ahn and S. Choi, “Design and implementation of low-profile contactless battery charger using planar printed circuit board windings as energy transfer device,” IEEE Trans. Industrial Electronics, vol. 51, no. 1, pp. 140-147, (February 2004); Y. Jang and M. M. Jovanovic, “A contactless electrical energy transmission system for portable-telephone battery chargers,” IEEE Trans. Industrial Electronics, vol. 50, no. 3, pp. 520-527, (June 2003); C.-G. Kim, D. H. Seo, J. S. You, J. H. Park and B. H. Cho, “Design of a contactless battery charger for cellular phone,” IEEE Trans. Industrial Electronics, vol. 48, no. 6, pp. 1238-1247, (December 2001); S. Y. R. Hui and W. C. Ho, “A new generation of universal contactless battery charging platform for portable Consumer Electronic equipment,” IEEE Trans. Power Electronics, vol. 20, no. 3, pp. 620-627, (May 2005); X. Liu and S. Y. R. Hui, “Simulation Study and Experimental Verification of a Contactless Battery Charging Platform with Localized Charging Features,” IEEE Trans. Power Electronics, vol. 22, no. 6, pp. 2202-2210, (November 2007); and S. Y. R. Hui, “Planar Inductive Battery Charging System”, U.S. Pat. No. 7,576,514, 2009. Wireless charging technology for portable electronic devices has reached the commercialization stage through the launch of the “Qi” Standard by the Wireless Power Consortium, now comprising over 135 companies worldwide. See the Wireless Power Consortium Website, available at: http://www.wirelesspowerconsortium.com.
The launch of the world first wireless power standard Qi by the Wireless Power Consortium for portable electronics products has sped up research and development activities in wireless power transfer. Recent wireless power research activities focus on both short-range applications and mid-range applications. In general, wireless power transfer systems (WPTS) can be classified as 2-coil systems, 4-coil systems, systems with relay resonators and wireless power domino-resonator systems. S. Y. R. Hui, W. X. Zhong and C. K. Lee, “A critical review on recent progress of mid-range wireless power transfer,” IEEE Transactions on Power Electronics (in press)
At present, a great deal of research is focused on improving the performance of wireless power transfer systems in order to increase the transfer distance, improve the efficiency and widen the operating frequency. All of these purposes are based on one thing, a well-known wireless power transfer system in which we know the topology of the system, all of the parameters of the components, the characteristics of the load, and the positions and directions of each coil. If so, it is easy to find the maximum efficiency operating point or maximum power transfer point, or the optimal operating point for other purposes. The biggest difficulty is how to find out the exact value of all the parameters of a given system, since as an extremely high order system, the slight difference between the predicted value and the real value of the parameters may lead to totally different performance at a given operating point, and some of these parameters cannot be measured precisely in an easy way.
On the other hand, even if we know all the parameters of the WPT system, the load may be dynamic and will change at any time. In order to operate the system always at the optimal point, monitoring of the impedance of the load at real time is required. Previously, a research team reported the use of a wireless communication method to transmit the load conditions as feedback information to the input power controller. N. Y. Kim, K. Y. Kim, J. Choi and C. W. Kim, “Adaptive frequency with power-level tracking system for efficient magnetic resonance wireless power transfer,” Electronics Letters, Vol. 48, No. 8, (April 2012), page(s): 452-454. This is a traditional approach that needs a wireless communication system, which may increase the cost of such overall system. In addition, the reported method does not involve any system parameter identification.